SPU: don't allocate SPU LS in vm::main

Create its own shared memory object.
Use vm::spu to allocate all SPU types.
Use vm::writer_lock for shm::map_critical.

rpcs3/Emu/Memory/vm.cpp

@@ -1313,10 +1313,10 @@ namespace vm
 			return {addr, nullptr};
 		}
 
-		// Special path
+		// Special case
 		if (m_common)
 		{
-			return {this->addr, m_common};
+			return {addr, nullptr};
 		}
 
 		// Range check